Asymmetrical tin-coupled rubbery polymers

ABSTRACT

Tire rubbers which are prepared by anionic polymerization are frequently coupled with a suitable coupling agent, such as a tin halide, to improve desired properties. It has been unexpectedly found that greatly improved properties for tire rubbers, such as lower hysteresis, can be attained by asymmetrically coupling the rubber. This invention more specifically discloses an asymmetrical tin-coupled rubbery polymer which is particularly valuable for use in manufacturing tire tread compounds, said asymmetrical tin-coupled rubbery polymer being comprised of a tin atom having at least three polydiene arms covalently bonded thereto, wherein said polydiene arms have a weight average molecular weight which is within the range of about 80,000 to about 300,000, wherein the polydiene arms do not contain blocks of more than about 4 repeat units of vinyl aromatic monomers, and wherein the ratio of the weight average molecular weight to the number average molecular weight of the asymmetrical tin-coupled rubbery polymer is within the range of about 1.4 to about 2.0.

BACKGROUND OF THE INVENTION

[0001] Tin-coupled polymers are known to provide desirable properties,such as improved treadwear and reduced rolling resistance, when used intire tread rubbers. Such tin-coupled rubbery polymers are typically madeby coupling the rubbery polymer with a tin coupling agent at or near theend of the polymerization used in synthesizing the rubbery polymer. Inthe coupling process, live polymer chain ends react with the tincoupling agent thereby coupling the polymer. For instance, up to fourlive chain ends can react with tin tetrahalides, such as tintetrachloride, thereby coupling the polymer chains together.

[0002] The coupling efficiency of the tin coupling agent is dependant onmany factors, such as the quantity of live chain ends available forcoupling and the quantity and type of polar modifier, if any, employedin the polymerization. For instance, tin coupling agents are generallynot as effective in the presence of polar modifiers. The amount ofcoupling which is attained is also, of course, highly dependent upon thequantity of tin coupling agent employed.

[0003] Each tin tetrahalide molecule is capable of reacting with up tofour live polymer chain ends. However, since perfect stoichiometry isdifficult to attain, some of the tin halide molecules often react withless than four live polymer chain ends. For instance, if more than astoichiometric amount of the tin halide coupling agent is employed, thenthere will be an insufficient quantity of live polymer chain ends tototally react with the tin halide molecules on a four to one basis. Onthe other hand, if less than a stoichiometric amount of the tin halidecoupling agent is added, then there will be an excess of live polymerchain ends and some of the live chain ends will not be coupled.

[0004] Conventional tin coupling results in the formation of a coupledpolymer which is essentially symmetrical. In other words, all of thepolymer arms on the coupled polymer are of essentially the same chainlength. All of the polymer arms in such conventional tin-coupledpolymers are accordingly of essentially the same molecular weight. Thisresults in such conventional tin-coupled polymers having a lowpolydispersity. For instance, conventional tin-coupled polymers normallyhaving a ratio of weight average molecular weight to number averagemolecular weight which is within the range of about 1.01 to about 1.1.

SUMMARY OF THE INVENTION

[0005] This invention is based upon the unexpected finding that greatlyimproved properties for tire rubbers, such as lower hysteresis, can beattained by asymmetrically coupling the rubber. For instance, suchasymmetrically coupled polymers can be utilized in making tires havinggreatly improved rolling resistance without sacrificing other tireproperties. These improved properties are due in part to betterinteraction and compatibility with carbon black. The asymmetrical tincoupling also normally leads to improve the cold flow characteristics ofthe rubbery polymer. Tin coupling in general also leads to betterprocessability and other beneficial properties.

[0006] The asymmetrical tin-coupled rubbery polymers of this inventionare comprised of a tin atom having polydiene arms covalently bondedthereto. At least one of the polydiene arms bonded to the tin atom willbe a low number molecular weight arm having a number average molecularweight of less than about 40,000. It is also critical for theasymmetrical tin-coupled rubbery polymer to have at least one highmolecular weight polydiene arm bonded to the tin atom. This highmolecular weight arm will have a number average molecular weight whichis at least 80,000. The ratio of the weight average molecular weight tothe number average molecular weight of the asymmetrical tin-coupledrubbery polymers of this invention will also be within the range ofabout 2 to about 2.5.

[0007] This invention more specifically discloses an asymmetricaltin-coupled rubbery polymer which is particularly valuable for use inmanufacturing tire tread compounds, said asymmetrical tin-coupledrubbery polymer being comprised of a tin atom having at least threepolydiene arms covalently bonded thereto, wherein at least one of saidpolydiene arms has a number average molecular weight of less than about40,000, wherein at least one of said polydiene arms has a number averagemolecular weight of at least about 80,000, and wherein the ratio of theweight average molecular weight to the number average molecular weightof the asymmetrical tin-coupled rubbery polymer is within the range ofabout 2 to about 2.5.

[0008] This invention also discloses an asymmetrical tin-coupled rubberypolymer which is particularly valuable for use in manufacturing tiretread compounds, said asymmetrical tin-coupled rubbery polymer beingcomprised of a tin atom having at least three polydiene arms covalentlybonded thereto, wherein at least one of said polydiene arms has a numberaverage molecular weight of less than about 40,000, wherein at least oneof said polydiene arms has a number average molecular weight of at leastabout 80,000, wherein the polydiene arms do not contain blocks of morethen about 4 repeat units of vinyl aromatic monomers, and wherein theratio of the weight average molecular weight to the number averagemolecular weight of the asymmetrical tin-coupled rubbery polymer iswithin the range of about 2 to about 2.5.

[0009] This invention also reveals a process for preparing anasymmetrical tin-coupled rubbery polymer which comprises: (1)continuously polymerizing at least one diene monomer to a conversion ofat least about 90 percent utilizing an anionic initiator to produce apolymer cement containing living polydiene rubber chains, wherein someof the living polydiene rubber chains are low molecular weight polydienerubber chains having a number average molecular weight of less thanabout 40,000, and wherein some of the living polydiene rubber chains arehigh molecular weight polydiene rubber chains having a number averagemolecular weight of greater than about 80,000; and (2) continuouslyadding a tin halide to the polymer cement in a separate reaction vesselto produce the asymmetrically tin-coupled rubbery polymer, wherein saidasymmetrical tin-coupled rubbery polymer has a polydispersity which iswithin the range of about 2 to about 2.5.

[0010] The present invention further discloses an asymmetricaltin-coupled rubbery polymer which is particularly valuable for use inmanufacturing tire tread compounds, said asymmetrical tin-coupledrubbery polymer being comprised of a tin atom having at least threepolydiene arms covalently bonded thereto, wherein said polydiene armshave a weight average molecular weight which is within the range ofabout 80,000 to about 300,000, wherein the polydiene arms do not containblocks of more than about 4 repeat units of vinyl aromatic monomers, andwherein the ratio of the weight average molecular weight to the numberaverage molecular weight of the asymmetrical tin-coupled rubbery polymeris within the range of about 1.4 to about 2.0.

[0011] The stability of the asymmetrical tin-coupled rubbery polymers ofthis invention can be improved by adding a tertiary chelating aminethereto subsequent to the time at which the tin-coupled rubbery polymeris coupled. N,N,N′,N′-tetramethylethylenediamine (TMEDA) is arepresentative example of a tertiary chelating amine which is preferredfor utilization in stabilizing the polymers of this invention.

DETAILED DESCRIPTION OF THE INVENTION

[0012] Virtually any type of rubbery polymer prepared by anionicpolymerization can be asymmetrically tin-coupled in accordance with thisinvention. The rubbery polymers that can be asymmetrically coupled willtypically be synthesized by a solution polymerization techniqueutilizing an organolithium compound as the initiator. These rubberypolymers will accordingly normally contain a “living” lithium chain end.

[0013] The polymerizations employed in synthesizing the living rubberypolymers will normally be carried out in a hydrocarbon solvent which canbe one or more aromatic, paraffinic or cycloparaffinic compounds. Thesesolvents will normally contain from 4 to 10 carbon atoms per moleculeand will be liquid under the conditions of the polymerization. Somerepresentative examples of suitable organic solvents include pentane,isooctane, cyclohexane, methylcyclohexane, isohexane, n-heptane,n-octane, n-hexane, benzene, toluene, xylene, ethylbenzene,diethylbenzene, isobutylbenzene, petroleum ether, kerosene, petroleumspirits, petroleum naphtha, and the like, alone or in admixture.

[0014] In the solution polymerization, there will normally be from 5 to30 weight percent monomers in the polymerization medium. Suchpolymerization media are, of course, comprised of the organic solventand monomers. In most cases, it will be preferred for the polymerizationmedium to contain from 10 to 25 weight percent monomers. It is generallymore preferred for the polymerization medium to contain 15 to 20 weightpercent monomers.

[0015] The rubbery polymers which are asymmetrically coupled inaccordance with this invention can be made by the homopolymerization ofa conjugated diolefin monomer or by the copolymerization of a conjugateddiolefin monomer with a vinyl aromatic monomer. It is, of course, alsopossible to make living rubbery polymers which can be asymmetricallytin-coupled by polymerizing a mixture of conjugated diolefin monomerswith one or more ethylenically unsaturated monomers, such as vinylaromatic monomers. The conjugated diolefin monomers which can beutilized in the synthesis of rubbery polymers which can beasymmetrically tin-coupled in accordance with this invention generallycontain from 4 to 12 carbon atoms. Those containing from 4 to 8 carbonatoms are generally preferred for commercial purposes. For similarreasons, 1,3-butadiene and isoprene are the most commonly utilizedconjugated diolefin monomers. Some additional conjugated diolefinmonomers that can be utilized include 2,3-dimethyl-1,3-butadiene,piperylene, 3-butyl-1,3-octadiene, 2-phenyl-1,3-butadiene, and the like,alone or in admixture.

[0016] Some representative examples of ethylenically unsaturatedmonomers that can potentially be synthesized into rubbery polymers whichcan be asymmetrically tin-coupled in accordance with this inventioninclude alkyl acrylates, such as methyl acrylate, ethyl acrylate, butylacrylate, methyl methacrylate and the like; vinylidene monomers havingone or more terminal CH2═CH— groups; vinyl aromatics such as styrene,α-methylstyrene, bromostyrene, chlorostyrene, fluorostyrene and thelike; α-olefins such as ethylene, propylene, 1-butene and the like;vinyl halides, such as vinylbromide, chloroethane (vinylchloride),vinylfluoride, vinyliodide, 1,2-dibromoethene, 1,1-dichloroethene(vinylidene chloride), 1,2-dichloroethene and the like; vinyl esters,such as vinyl acetate; α,β-olefinically unsaturated nitrites, such asacrylonitrile and methacrylonitrile; α,β-olefinically unsaturatedamides, such as acrylamide, N-methyl acrylamide, N,N-dimethylacrylamide,methacrylamide and the like.

[0017] Rubbery polymers which are copolymers of one or more dienemonomers with one or more other ethylenically unsaturated monomers willnormally contain from about 50 weight percent to about 99 weight percentconjugated diolefin monomers and from about 1 weight percent to about 50weight percent of the other ethylenically unsaturated monomers inaddition to the conjugated diolefin monomers. For example, copolymers ofconjugated diolefin monomers with vinylaromatic monomers, such asstyrene-butadiene rubbers which contain from 50 to 99 weight percentconjugated diolefin monomers and from 1 to 50 weight percentvinylaromatic monomers, are useful in many applications.

[0018] Vinyl aromatic monomers are probably the most important group ofethylenically unsaturated monomers which are commonly incorporated intopolydienes. Such vinyl aromatic monomers are, of course, selected so asto be copolymerizable with the conjugated diolefin monomers beingutilized. Generally, any vinyl aromatic monomer which is known topolymerize with organolithium initiators can be used. Such vinylaromatic monomers typically contain from 8 to 20 carbon atoms. Usually,the vinyl aromatic monomer will contain from 8 to 14 carbon atoms. Themost widely used vinyl aromatic monomer is styrene. Some examples ofvinyl aromatic monomers that can be utilized include styrene,1-vinylnaphthalene, 2-vinylnaphthalene, a-methylstyrene,4-phenylstyrene, 3-methylstyrene and the like. In most cases the rubberwill contain no more than about 25 weight percent of the vinylaromaticmonomer. It is critical for such vinyl aromatic monomers to bedistributed randomly throughout the polymer chains of such rubberypolymers. Thus, a modifier such as sodium dodecylbenzene sulfonate willbe employed during the polymerization to insure that repeat units whichare derived from the vinyl aromatic monomers are randomly distributedthroughout the polymer chains.

[0019] Some representative examples of rubbery polymers which can beasymmetrically tin-coupled in accordance with this invention includepolybutadiene, polyisoprene, random styrene-butadiene rubber (SBR),random a-methylstyrene-butadiene rubber, random α-methylstyrene-isoprenerubber, random styrene-isoprene-butadiene rubber (SIBR), randomstyrene-isoprene rubber (SIR), random isoprene-butadiene rubber (IBR),random a-methylstyrene-isoprene-butadiene rubber and randoma-methylstyrene-styrene-isoprene-butadiene rubber.

[0020] The polymerizations employed in making the rubbery polymer aretypically initiated by adding an organolithium initiator to an organicpolymerization medium which contains the monomers. Such polymerizationsare typically carried out utilizing continuous polymerizationtechniques. In such continuous polymerizations, monomers and initiatorare continuously added to the organic polymerization medium with therubbery polymer synthesized being continuously withdrawn. Suchcontinuous polymerizations are typically conducted in a multiple reactorsystem.

[0021] The organolithium initiators which can be employed insynthesizing rubbery polymers which can be asymmetrically coupled inaccordance with this invention include the monofunctional andmultifunctional types known for polymerizing the monomers describedherein. The multifunctional organolithium initiators can be eitherspecific organolithium compounds or can be multifunctional types whichare not necessarily specific compounds but rather represent reproduciblecompositions of regulable functionality.

[0022] The amount of organolithium initiator utilized will vary with themonomers being polymerized and with the molecular weight that is desiredfor the polymer being synthesized. However, as a general rule, from 0.01to 1 phm (parts per 100 parts by weight of monomer) of an organolithiuminitiator will be utilized. In most cases, from 0.01 to 0.1 phm of anorganolithium initiator will be utilized with it being preferred toutilize 0.025 to 0.07 phm of the organolithium initiator.

[0023] The choice of initiator can be governed by the degree ofbranching and the degree of elasticity desired for the polymer, thenature of the feedstock and the like. With regard to the feedstockemployed as the source of conjugated diene, for example, themultifunctional initiator types generally are preferred when a lowconcentration diene stream is at least a portion of the feedstock, sincesome components present in the unpurified low concentration diene streammay tend to react with carbon lithium bonds to deactivate initiatoractivity, thus necessitating the presence of sufficient lithiumfunctionality in the initiator so as to override such effects.

[0024] The multifunctional initiators which can be used include thoseprepared by reacting an organomonolithium compounded with amultivinylphosphine or with a multivinylsilane, such a reactionpreferably being conducted in an inert diluent such as a hydrocarbon ora mixture of a hydrocarbon and a polar organic compound. The reactionbetween the multivinylsilane or multivinylphosphine and theorganomonolithium compound can result in a precipitate which can besolubilized, if desired, by adding a solubilizing monomer such as aconjugated diene or monovinyl aromatic compound, after reaction of theprimary components. Alternatively, the reaction can be conducted in thepresence of a minor amount of the solubilizing monomer. The relativeamounts of the organomonolithium compound and the multivinylsilane orthe multivinylphosphine preferably should be in the range of about 0.33to 4 moles of organomonolithium compound per mole of vinyl groupspresent in the multivinylsilane or multivinylphosphine employed. Itshould be noted that such multifunctional initiators are commonly usedas mixtures of compounds rather than as specific individual compounds.

[0025] Exemplary organomonolithium compounds include ethyllithium,isopropyllithium, n-butyllithium, sec-butyllithium, tert-octyllithium,n-eicosyllithium, phenyllithium, 2-naphthyllithium,4-butylphenyllithium, 4-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium and the like.

[0026] Exemplary multivinylsilane compounds include tetravinylsilane,methyltrivinylsilane, diethyldivinylsilane, di-n-dodecyldivinylsilane,cyclohexyltrivinylsilane, phenyltrivinylsilane, benzyltrivinylsilane,(3-ethylcyclohexyl) (3-n-butylphenyl)divinylsilane and the like.

[0027] Exemplary multivinylphosphine compounds includetrivinylphosphine, methyldivinylphosphine, dodecyldivinylphosphine,phenyldivinylphosphine, cyclooctyldivinylphosphine and the like.

[0028] Other multifunctional polymerization initiators can be preparedby utilizing an organomonolithium compound, further together with amultivinylaromatic compound and either a conjugated diene ormonovinylaromatic compound or both. These ingredients can be chargedinitially, usually in the presence of a hydrocarbon or a mixture of ahydrocarbon and a polar organic compound as a diluent. Alternatively, amultifunctional polymerization initiator can be prepared in a two-stepprocess by reacting the organomonolithium compound with a conjugateddiene or monovinyl aromatic compound additive and then adding themultivinyl aromatic compound. Any of the conjugated dienes or monovinylaromatic compounds described can be employed. The ratio of conjugateddiene or monovinyl aromatic compound additive employed preferably shouldbe in the range of about 2 to 15 moles of polymerizable compound permole of organolithium compound. The amount of multivinylaromaticcompound employed preferably should be in the range of about 0.05 to 2moles per mole of organomonolithium compound.

[0029] Exemplary multivinyl aromatic compounds include1,2-divinylbenzene, 1,3-divinylbenzene, 1,4-divinylbenzene,1,2,4-trivinylbenzene, 1,3-divinylnaphthalene, 1,8-divinylnaphthalene,1,3,5-trivinylnaphthalene, 2,4-divinylbiphenyl, 3,5,4′-trivinylbiphenyl,m-diisopropenyl benzene, p-diisopropenyl benzene,1,3-divinyl-4,5,8-tributylnaphthalene and the like. Divinyl aromatichydrocarbons containing up to 18 carbon atoms per molecule arepreferred, particularly divinylbenzene as either the ortho, meta or paraisomer and commercial divinylbenzene, which is a mixture of the threeisomers, and other compounds, such as the ethylstyrenes, also is quitesatisfactory.

[0030] Other types of multifunctional initiators can be employed such asthose prepared by contacting a sec- or tert-organomonolithium compoundwith 1,3-butadiene, at a ratio of about 2 to 4 moles of theorganomonolithium compound per mole of the 1,3-butadiene, in the absenceof added polar material in this instance, with the contacting preferablybeing conducted in an inert hydrocarbon diluent, though contactingwithout the diluent can be employed if desired.

[0031] Alternatively, specific organolithium compounds can be employedas initiators, if desired, in the preparation of polymers in accordancewith the present invention. These can be represented by R(Li)x wherein Rrepresents a hydrocarbyl radical containing from 1 to 20 carbon atoms,and wherein x is an integer of 1 to 4. Exemplary organolithium compoundsare methyllithium, isopropyllithium, n-butyllithium, sec-butyllithium,tert-octyllithium, n-decyllithium, phenyllithium, 1-naphthyllithium,4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium, 4-butylcyclohexyllithium, 4-cyclohexylbutyllithium,dilithiomethane, 1,4-dilithiobutane, 1,10-dilithiodecane,1,20-dilithioeicosane, 1,4-dilithiocyclohexane, 1,4-dilithio-2-butane,1,8-dilithio-3-decene, 1,2-dilithio-1,8-diphenyloctane,1,4-dilithiobenzene, 1,4-dilithionaphthalene, 9,10-dilithioanthracene,1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane,1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane,1,3,5,8-tetralithiodecane, 1,5,10,20-tetralithioeicosane,1,2,4,6-tetralithiocyclohexane, 4,4′-dilithiobiphenyl and the like.

[0032] The polymerization temperature utilized can vary over a broadrange of from about −20□ C. to about 180□ C. In most cases, atemperature within the range of about 30□ C. to about 125□ C. will beutilized. It is typically most preferred for the polymerizationtemperature to be within the range of about 60□ C. to about 85□ C. Thepressure used will normally be sufficient to maintain a substantiallyliquid phase under the conditions of the polymerization reaction.

[0033] The polymerization is conducted for a length of time sufficientto permit substantially complete polymerization of monomers. In otherwords, the polymerization is normally carried out until high conversionsare attained. The polymerization is then terminated by the continuousaddition of a tin coupling agent. This continuous addition of tincoupling agent is normally done in a reaction zone separate from thezone where the bulk of the polymerization is occurring. In other words,the tin coupling agent will typically be added only after a high degreeof conversion has already been attained. For instance, the tin couplingagent will normally be added only after a monomer conversion of greaterthan about 90 percent has been realized. It will typically be preferredfor the monomer conversion to reach at least about 95 percent before thetin coupling agent is added. As a general rule, it is most preferred forthe monomer conversion to exceed about 98 percent before the tincoupling agent is added. The tin coupling agent will normally be addedin a separate reaction vessel after the desired degree of conversion hasbeen attained. The tin coupling agent can be added in a hydrocarbonsolution, e.g., in cyclohexane, to the polymerization admixture withsuitable mixing for distribution and reaction.

[0034] The tin coupling agent will normally be a tin tetrahalide, suchas tin tetrachloride, tin tetrabromide, tin tetrafluoride or tintetraiodide. However, tin trihalides can also optionally be used. Incases where tin trihalides are utilized, a coupled polymer having amaximum of three arms results. To induce a higher level of branching,tin tetrahalides are normally preferred. As a general rule, tintetrachloride is most preferred.

[0035] Broadly, and exemplary, a range of about 0.01 to 4.5milliequivalents of tin coupling agent is employed per 100 grams of therubbery polymer. It is normally preferred to utilize about 0.01 to about1.5 milliequivalents of the tin coupling agent per 100 grams of polymerto obtain the desired Mooney viscosity. The larger quantities tend toresult in production of polymers containing terminally reactive groupsor insufficient coupling. One equivalent of tin coupling agent perequivalent of lithium is considered an optimum amount for maximumbranching. For instance, if a tin tetrahalide is used as the couplingagent, one mole of the tin tetrahalide would be utilized per four molesof live lithium ends. In cases where a tin trihalide is used as thecoupling agent, one mole of the tin trihalide will optimally be utilizedfor every three moles of live lithium ends. The tin coupling agent canbe added in a hydrocarbon solution, e.g., in cyclohexane, to thepolymerization admixture in the reactor with suitable mixing fordistribution and reaction.

[0036] After the tin coupling has been completed, a tertiary chelatingalkyl 1,2-ethylene diamine can optionally be added to the polymer cementto stabilize the asymmetrically tin-coupled rubbery polymer. Thetertiary chelating amines which can be used are normally chelating alkyldiamines of the structural formula:

[0037] wherein n represents an integer from 1 to about 6, wherein Arepresents an alkylene group containing from 1 to about 6 carbon atomsand wherein R₁, R₂, R₃ and R₄ can be the same or different and representalkyl groups containing from 1 to about 6 carbon atoms. The alkylenegroup A is the formula —(—CH₂—)_(m) wherein m is an integer from 1 toabout 6. The alkylene group will typically contain from I to 4 carbonatoms (m will be 1 to 4) and will preferably contain 2 carbon atoms. Inmost cases, n will be an integer from 1 to about 3 with it beingpreferred for n to be 1. It is preferred for R₁, R₂, R₃ and R₄ torepresent alkyl groups which contain from 1 to 3 carbon atoms. In mostcases, R₁, R2, R₃ and R₄ will represent methyl groups.

[0038] A sufficient amount of the chelating amine should be added tocomplex with any residual tin coupling agent remaining after completionof the coupling reaction.

[0039] In most cases, from about 0.01 phr (parts by weight per 100 partsby weight of dry rubber) to about 2 phr of the chelating alkyl1,2-ethylene diamine will be added to the polymer cement to stabilizethe rubbery polymer. Typically, from about 0.05 phr to about 1 phr ofthe chelating alkyl 1,2-ethylene diamine will be added. More typically,from about 0.1 phr to about 0.6 phr of the chelating alkyl 1,2-ethylenediamine will be added to the polymer cement to stabilize the rubberypolymer.

[0040] After the polymerization, asymmetrical tin coupling, andoptionally the stabilization step, has been completed, the asymmetricaltin-coupled rubbery polymer can be recovered from the organic solvent.The asymmetrical tin-coupled rubbery polymer can be recovered from theorganic solvent and residue by means such as decantation, filtration,centrification and the like. It is often desirable to precipitate theasymmetrically tin-coupled rubbery polymer from the organic solvent bythe addition of lower alcohols containing from about 1 to about 4 carbonatoms to the polymer solution. Suitable lower alcohols for precipitationof the rubber from the polymer cement include methanol, ethanol,isopropyl alcohol, normal-propyl alcohol and t-butyl alcohol. Theutilization of lower alcohols to precipitate the asymmetricallytin-coupled rubbery polymer from the polymer cement also “kills” anyremaining living polymer by inactivating lithium end groups. After theasymmetrically tin-coupled rubbery polymer is recovered from thesolution, steam-stripping can be employed to reduce the level ofvolatile organic compounds in the asymmetrically tin-coupled rubberypolymer.

[0041] The asymmetrical tin-coupled rubbery polymers of this inventionare comprised of a tin atom having at least three polydiene armscovalently bonded thereto. At least one of the polydiene arms bonded tothe tin atom has a number average molecular weight of less than about40,000 and at least one of the polydiene arms bonded to the tin atom hasa number average molecular weight of at least about 80,000. The ratio ofthe weight average molecular weight to the number average molecularweight of the asymmetrical tin-coupled rubbery polymer will also bewithin the range of about 1.4 to about 2.5.

[0042] The asymmetrical tin-coupled rubbery polymers of this inventionare of the structural formula:

[0043] wherein R¹, R², R³ and R⁴ can be the same or different and areselected from the group consisting of alkyl groups and polydiene arms(polydiene rubber chains), with the proviso that at least three membersselected from the group consisting of R¹, R², R3 and R⁴ are polydienearms, with the proviso that at least one member selected from the groupconsisting of R¹, R², R³ and R⁴ is a low molecular weight polydiene armhaving a number average molecular weight of less than about 40,000, withthe proviso that at least one member selected from the group consistingof R¹, R², R³ and R⁴ is a high molecular weight polydiene arm having anumber average molecular weight of greater than about 80,000, and withthe proviso that the ratio of the weight average molecular weight to thenumber average molecular weight of the asymmetrical tin-coupled rubberypolymer is within the range of about 1.4 to about 2.5. It should benoted that R¹, R², R³ and R⁴ can be alkyl groups because it is possiblefor the tin halide coupling agent to react directly with alkyl lithiumcompounds which are used as the polymerization initiator.

[0044] In most cases, four polydiene arms will be covalently bonded tothe tin atom in the asymmetrical tin-coupled rubbery polymer. In suchcases, R¹, R², R³ and R⁴ will all be polydiene arms. The asymmetricaltin-coupled rubbery polymer will often contain a polydiene arm ofintermediate molecular weight as well as the low molecular weight armand the high molecular weight arm. Such intermediate molecular weightarms will have a molecular weight that is within the range of about45,000 to about 75,000. It is normally preferred for the low molecularpolydiene arm to have a molecular weight of less than about 30,000 withit being most preferred for the low molecular weight arm to have amolecular weight of less than about 25,000. It is normally preferred forthe high molecular polydiene arm to have a molecular weight of greaterthan about 90,000 with it being most preferred for the high molecularweight arm to have a molecular weight of greater than about 100,000.

[0045] The polydiene arms in the asymmetrical tin-coupled rubberypolymers of this invention will typically have a weight averagemolecular weight that is within the range of about 80,000 to about300,000. The polydiene arms will more typically have a weight averagemolecular weight that is within the range of about 120,000 to about250,000. The ratio of the weight average molecular weight to the numberaverage molecular weight of the asymmetrical tin-coupled rubbery polymeris typically within the range of about 1.4 to about 2.5. The ratio ofthe weight average molecular weight to the number average molecularweight of the asymmetrical tin-coupled rubbery polymer is typicallywithin the range of about 1.4 to about 2.0. The ratio of the weightaverage molecular weight to the number average molecular weight of theasymmetrical tin-coupled rubbery polymer is more typically within therange of about 1.45 to about 1.8. The ratio of the weight averagemolecular weight to the number average molecular weight of theasymmetrical tin-coupled rubbery polymer is most typically within therange of about 1.5 to about 2.7. Normally at least one of the polydienearms covalently bonded to the tin has a weight average molecular weightof less than about 150,000 and at least one of the polydiene armscovalently bonded to the tin atom has a weight average molecular weightof greater than about 200,000.

[0046] The polydiene arms in the asymmetrical tin-coupled rubberypolymers of this invention are not block copolymers. In cases wherevinyl aromatic monomers, such as styrene, are present in the polydienearms they are not present in blocks containing more than about 4 repeatunits. In other words, the vinyl aromatic monomers will be distributedin a random fashion throughout the polydiene arms.

[0047] This invention is illustrated by the following examples which aremerely for the purpose of illustration and are not to be regarded aslimiting the scope of the invention or the manner in which it can bepracticed. Unless specifically indicated otherwise, parts andpercentages are given by weight.

[0048] Benefits with tin-coupled IBRs, as compared to the linear IBRs,are demonstrated by the following examples. These benefits include:

[0049] (1) Improvements in processability, particularlyextrudability/extrudate quality.

[0050] (2) Treadwear improvement and rolling resistance reduction due toimproved carbon black dispersion with the tin-coupled IBR. Gooddispersion of carbon black prevents carbon particles from forming anetwork of carbon black in the vulcanizate and reduces hysteresisresulting from carbon black aggregates. This is known as Payne effect.The higher the Payne effect, the better the carbon black dispersion. ThePayne effect can be measured as follows:${{Payne}\quad {effect}} = {\frac{G^{\prime}\quad {at}\quad 10\% \quad {strain}}{G^{\prime}\quad {at}\quad 1\% \quad {strain}} \times 100}$

EXAMPLE 1

[0051] In this example, a coupled isoprene-butadiene rubber (IBR) wasprepared in a one-gallon (3.8 liters) batch reactor at 70° C. In theprocedure used, 2,000 grams of a silica/molecular sieve/aluminum driedpremix containing 19.0 weight percent of a mixture of isoprene and1,3-butadiene in hexanes at the ratio of 10:90 was charged into aone-gallon (3.8 liters) reactor. After the amount of impurity in thepremix was determined, 4.0 ml of a 1.0 M solution of n-butyl lithium (inhexane) was added to the reactor. The target Mn (number averagedmolecular weight) was 100,000. The polymerization was allowed to proceedat 70° C. for three hours. An analysis of the residual monomer indicatedthat monomers was all consumed. Then, 1.0 ml of a 1 M solution of tintetrachloride (in hexane) was added to the reactor and the couplingreaction was carried out at the same temperature for 30 minutes. At thistime, 1.5 phr (parts per 100 parts by weight of rubber) of4-t-butylcatechol and 0.5 phr of TMEDA was added to the reactor toshortstop the polymerization and to stabilize the polymer.

[0052] After the hexane solvent was evaporated, the resulting SIBR wasdried in a vacuum oven at 50° C. The coupled IBR was determined to havea glass transition temperature (Tg) at −95° C. It was also determined tohave a microstructure which contained 7 percent 1,2-polybutadiene units,87 percent 1,4-polybutadiene units, 1 percent 3,4-polyisoprene units and9 percent 1,4-polyisoprene units. The Mooney viscosity (ML-4) of thecoupled IBR made was determined to be 99.

EXAMPLES 2-4

[0053] The procedure described in Example 1 was utilized in theseexamples except that the isoprene to 1,3-butadiene ratio were changedfrom 10:90 to 15:85, 20:80 and 30:70. The Tgs, Mooney viscosities (ML-4)and microstructures of these tin-coupled IBRs are listed in Table I. The30/70 IBR (Example 4) was determined to have an Mn (number averagedmolecular weight) of 386,000 and a Mw (weight averaged molecular weight)of 430,000. The precursor of Example 4 (ie, base polymer prior tocoupling) was also determined to have an Mn of 99,000 and an Mw of112,000. TABLE I Isoprene/Bd Tg Microstructure (%) Ex. Composition (°C.) ML-4 1,2-PBd 1,4-PBd 3,4-PI 1,4-PI 1 10/90 −95 99 7 83 2  8 2 15/85−93 91 8 77 1 14 3 20/80 −90 82 8 72 1 19 4 30/70 −87 84 7 63 3 27

EXAMPLES 5-8

[0054] In these examples, linear IBRs were prepared in a one-gallonreactor. The procedure described in Example 1 was utilized in theseexamples except that no coupling agent (tin tetrachloride) was used inthese experiments and the target Mn was changed to 300,000 from 100,000.The isoprene to 1,3-butadiene ratios were 10:90, 15:85, 20:80 and 30:70.The Tgs, Mooney viscosities (ML-4), Mns (number averaged molecularweights), Mws (weight averaged molecular weights) and microstructures ofthese linear IBRs are listed in Table II. TABLE II Isoprene/Bd TgMicrostructure (%) Ex. Composition (° C.) ML-4 Mn Mw 1,2-PBd 1,4-PBd3,4-PI 1,4-PI 1 10/90 −96 88 308 K 326 K 7 83 1  9 2 15/85 −94 81 307 K329 K 7 77 1 15 3 20/80 −92 82 317 K 338 K 7 72 1 20 4 30/70 −89 87 313K 332 K 6 62 2 30

EXAMPLE 9

[0055] The tin-coupled IBR prepared in this experiment was synthesizedin a three-reactor (10 gallons each) continuous system at 90° C. Apremix containing isoprene and 1,3-butadiene in hexane was charged intothe first reactor continuously at a rate of 65.6 grams/minute. Thepremix monomer solution containing a ratio of isoprene to 1,3-butadieneof 30:70 and had a total monomer concentration of 14 percent.Polymerization was initiated by adding 0.128 M solution of n-butyllithium into the first reactor at a rate of 0.4 grams per minute. Mostof the monomers were exhausted at the end of the second reactor and theresulted polymerization medium containing the live ends was continuouslypushed into the third reactor where the coupling agent, tintetrachloride, (0.025 M solution in hexane) was added at a rate of 0.34grams per minute. The residence time for all three reactors was set at1.5 hours to achieve complete monomer conversion in the second reactorand complete coupling in the third reactor. The polymerization mediumwas then continuously pushed over to a holding tank containing the TMEDAand an antioxidant. The resulting polymer cement was then steam-strippedand the recovered IBR was dried in an oven at 60° C. The polymer wasdetermined to have a glass transition temperature at −85° C. and have aMooney ML-4 viscosity of 90. It was also determined to have amicrostructure which contained 8 percent 1,2-polybutadiene units, 60percent 1,4-polybutadiene units, 29 percent 1,4-polyisoprene units and 3percent 3,4-polyisoprene units. The polymer was determined to have a Mn(number averaged molecular weight) of 185,000 and a Mw (weight averagedmolecular weight) of 276,000. The precursor of this polymer (i.e., basepolymer prior to coupling) was also determined to have an Mn of 88,000and an Mw of 151,000.

[0056] Unlike the Example 4 (prepared and coupled in a batch process)which showed a symmetrical coupling of four linear precursor polymers,the polymer produced in this example via the continuous process hadunsymmetrical coupling base on GPC molecular data shown above.

EXAMPLE 10-12

[0057] The isoprene-butadiene rubbers made in Example 4, 8 and 9 werethen compounded utilizing a standard tire tread test formulation. Thetire tread test formulations were made by mixing 100 parts of rubberbeing tested with 50 parts of carbon black, 5 parts of processing oil, 2parts of stearic acid, 3 parts of zinc oxide, 1 part of microcrystallinewax, 0.5 part of paraffin wax, 1 part of a mixed aryl-p-phenylenediamineantioxidant, 2 parts of N-(1,3-dimethyl butyl)-N′-phenyl-p-phenylenediene and 1.4 parts of sulfur. The physical properties of the compoundedtire tread formulations are reported in Table III. TABLE III Example 8 49 Rubber Type Linear Batch continuous coupled coupled Rheometer, 150° C.ML, dNm 1.21 1.49 1.67 MH, dNm 24.06 25.71 23.71 ts1, min 4.06 4.86 5.39T25, min 5.62 5.78 6.50 T90, min 10.36 9.71 9.88 Autovibron, 11 Hz tandelta at 60□C 0.113 0.083 0.072 G′ at 10% 2.494 2.294 2.195 G′ at 1%3.435 2.732 2.566 Payne effect 72.6 84.4 85.5

[0058] Variations in the present invention are possible in light of thedescription of it provided herein. While certain representativeembodiments and details have been shown for the purpose of illustratingthe subject invention, it will be apparent to those skilled in this artthat various changes and modifications can be made therein withoutdeparting from the scope of the subject invention. It is, therefore, tobe understood that changes can be made in the particular embodimentsdescribed which will be within the full intended scope of the inventionas defined by the following appended claims.

What is claimed is:
 1. An asymmetrical tin-coupled rubbery polymer whichis particularly valuable for use in manufacturing tire tread compounds,said asymmetrical tin-coupled rubbery polymer being comprised of a tinatom having at least three polydiene arms covalently bonded thereto,wherein said polydiene arms have a weight average molecular weight whichis within the range of about 80,000 to about 300,000, wherein thepolydiene arms do not contain blocks of more than about 4 repeat unitsof vinyl aromatic monomers, and wherein the ratio of the weight averagemolecular weight to the number average molecular weight of theasymmetrical tin-coupled rubbery polymer is within the range of about1.4 to about 2.0.
 2. An asymmetrical tin-coupled rubbery polymer asspecified in claim 1 wherein the polydiene arms are comprised of amember selected from the group consisting of random styrene-polyisoprenechains, random styrene-butadiene chains, randomα-methylstyrene-butadiene chains, random α-methylstyrene-isoprenechains, random styrene-isoprene-butadiene chains, randomα-methylstyrene-isoprene-butadiene chains and randomα-methylstyrene-styrene-isoprene-butadiene chains.
 3. An asymmetricaltin-coupled rubbery polymer as specified in claim 2 wherein 4 polydienearms are covalently bonded to the tin atom.
 4. An asymmetricaltin-coupled rubbery polymer as specified in claim 3 wherein the ratio ofthe weight average molecular weight to the number average molecularweight of the asymmetrical tin-coupled rubbery polymer is within therange of about 1.45 to about 1.8.
 5. An asymmetrical tin-coupled rubberypolymer as specified in claim 4 wherein said polydiene arms have aweight average molecular weight which is within the range of about120,000 to about 250,000
 6. An asymmetrical tin-coupled rubbery polymeras specified in claim 5 wherein the ratio of the weight averagemolecular weight to the number average molecular weight of theasymmetrical tin-coupled rubbery polymer is within the range of about1.5 to about 1.7.
 7. An asymmetrical tin-coupled rubbery polymer asspecified in claim 6 wherein at least one of the polydiene armscovalently bonded to the tin atom has a weight average molecular weightof less than about 150,000.
 8. An asymmetrical tin-coupled rubberypolymer as specified in claim 7 wherein at least one of the polydienearms covalently bonded to the tin atom has a weight average molecularweight of greater than about 200,000.
 9. An asymmetrical tin-coupledrubbery polymer as specified in claim 8 wherein said polydiene arms arerandom styrene-isoprene-butadiene chains.
 10. An asymmetricaltin-coupled rubbery polymer as specified in claim 8 wherein saidpolydiene arms are random styrene-isoprene chains.
 11. An asymmetricaltin-coupled rubbery polymer as specified in claim 8 wherein saidpolydiene arms are random styrene-butadiene chains.